A Combined Muscle Model and Wavelet Approach to Interpreting the Surface EMG Signals from Maximal Dynamic Knee Extensions

Abstract

This study aimed to identify areas of reduced surface EMG amplitude and changed frequency across the phase space of a maximal dynamic knee extension task. The hypotheses were: (1) amplitude would be lower for eccentric contractions compared to concentric contractions and unaffected by fibre length; and (2) mean frequency would also be lower for eccentric contractions and unaffected by fibre length. Joint torque and EMG signals from the vastii and rectus femoris were recorded for eight athletic subjects performing maximum knee extensions at thirteen joint velocities spanning ±250° s -1 . The instantaneous amplitude and mean frequency were calculated using the continuous wavelet transform time -frequency method, and the fibre dynamics were determined using a muscle model of the knee extensions. The results indicated: (1) only for the rectus femoris were amplitudes significantly lower for eccentric contractions (p = 0.019), for the vastii amplitudes during eccentric contractions were less than maximal, but this was also the case for concentric contractions due to a significant reduction in amplitude towards knee extension (p = 0.023); and (2) mean frequency increased significantly with decreasing fibre length for all knee extensors and contraction velocities (p = 0.029). Using time -frequency processing of the EMG signals and a muscle model allowed the simultaneous assessment of fibre length, velocity and EMG.

The torque–velocity relationship in large human muscles: Maximum voluntary versus electrically stimulated behaviour

“…Maximum torque expressed at the joint level is a complex integration of the muscle fibre contractile properties with the in vivo architecture of multiple muscle fibres, connective tissue and neural input. In vivo measurements of maximum voluntary contraction's (MVC) forcevelocity show differences to the in vitro tetanic profile, with eccentric forces not increasing much above isometric and tending to decline with increasing lengthening velocity (Westing et al, 1988;Dudley et al, 1990;Weber and Kriellaars, 1997;Kellis and Baltzopoulos, 1998;Forrester and Pain, 2010). Consequently, maximum voluntary eccentric strength is much lower than one might expect based on maximum isometric measurements and in vitro tetanic force-velocity behaviour.…”

Contraction type influences the human ability to use the available torque capacity of skeletal muscle during explosive efforts

“…Earlier studies have also reported reduced agonist activation during voluntary contractions at more extended knee angles [4,20,30,31], and this effect is thought to be a neural mechanism that protects the knee joint near full extension, where loading of the anterior cruciate ligament is greatest [32].…”

An Isovelocity Dynamometer Method to Determine Monoarticular and Biarticular Muscle Parameters

“…Ltd = tendon slack length; width = width of force-length curve; L opt = optimal fiber length; F 0 = maximum voluntary isometric force; V maI = maximum shortening velocity; F^ = maximum voluntary eccentric force; -V c = vertical asymptote of concentric hyperbola. The optimizations were performed in a two-stage process (isometric followed by dynamic) as described in Forrester et al (2011). Width (-) Note.…”

“…A series of isometric and isovelocity ankle plantar flexion trials were collected at six knee angles (168°, 145°, 135°, 125°, 110° and 45°, where 180° corresponded to a straight leg). These angles did not include the very extremes in the subject's range of motion to avoid possible neural inhibition effects (Forrester & Pain, 2010). For each knee angle, trials were performed at six isometric ankle angles covering the full range of motion and six repeated concentric-eccentric contractions at angular velocities between 50 and 300 7s.…”

“…Firstly, it was assumed that the triceps surae generated the entire plantar flexion moment; that is, contributions from the other plantar flexors and antagonist muscle activity were neglected. The triceps surae accounts for approximately 90% of the total positive plantar flexion torque capacity (Spoor et al, 1990;Ward et al, 2009), while previous EMG measurements during maximal voluntary dynamometer strength testing have reported antagonist muscle activity to be negligible provided that the subject is accustomed to maximal dynamometer testing (Winter & Challis, 2008;Forrester and Pain, 2010). Secondly, the medial and lateral heads of the gastrocnemius were combined.…”